The present invention discloses a class of water soluble block polymers of relatively low molecular weight, which have been found to efficiently viscosify water. These polymers have a water soluble block and a hydrocarbon soluble block. For aqueous solubility, the water soluble block is greater than 80 weight percent of the total polymer. One method of preparing the block polymers is to anionically polymerize a hydrocarbon soluble block polymer. This is followed by the selective functionalization of the major block to render said block water soluble. The functionalization in this invention is sulfonation followed by neutralization of the resulting sulfonic acid. These block polymers readily dissolve in water. The resulting solutions exhibit enhanced viscosity at low concentrations (i.e., 500 to 1500 ppm) relative to solutions of homopolymers of even higher molecular weight. They also have viscosity levels significantly greater than corresponding random copolymer solutions of similar composition.
Numerous U.S. Patents have taught methods of sulfonating a variety of elastomeric and thermoplastic polymers. These U.S. Pat. Nos. are: 3,642,728; 3,836,511; 3,072,618; 3,072,619; 3,847,854; 3,870,841 and 3,887,530. However, none of these references teach the process for preparing the sulfonated diblock and triblock polymers of the instant invention, nor do they recognize that sulfonated polymers of this class are excellent viscosification agents for aqueous solutions.